Effect of heat treatment on hot deformation behavior and microstructure

Effect of heat treatment on hot deformation behavior and microstructure evolution of 7085aluminum alloy

Songyi Chen a ,Kanghua Chen a ,?,Guosheng Peng a ,Xuehai Chen a ,Qinghua Ceng b

a State Key Laboratory of Powder Metallurgy,Central South University,Changsha 410083,China b

Southwest Aluminum (Group)Co.,Ltd.,Chongqing 401326,China

a r t i c l e i n f o Article history:

Received 10March 2012

Received in revised form 30April 2012Accepted 15May 2012

Available online 24May 2012Keywords:

7085Aluminum alloy Heat treatment

Hot deformation behavior Dynamic precipitation

a b s t r a c t

The hot deformation behaviors of 7085aluminum alloy treated by (a)homogenization and (b)solution treatment were studied by a series of compression testing over a temperature range from 300°C to 450°C with strain rate from 0.0001s à1to 1s à1.The results show the ?ow stresses are sensitive to the deformation parameters and heat treatment conditions.The peak ?ow stress decreases with increasing deformation temperature and decreasing strain rate.The peak ?ow stress of the alloy after solution treat-ment is signi?cantly higher than that of the alloy after homogenization treatment under the similar com-pression condition,especially at low deformation temperature.The peak ?ow stresses for the alloys after homogenization and solution treatment can be represented by the Zener–Hollomon parameter (Z )in the hyperbolic-sine equation with the hot deformation activation energy of 187.4kJ/mol and 318.9kJ/mol,respectively.The differences of both treated alloys on hot deformation behaviors were mainly attributed to precipitation characteristic.The ?ow stresses softening for the alloy after homogenization treatment is accounted for dynamic recovery and dynamic recrystallization.However,dynamic precipitation and coarsening as well as dynamic recovery are responsible for the ?ow stress softening at low deformation temperature while dynamic recovery and dynamic recrystallization are the main reasons for the ?ow stress softening at high deformation temperature for the alloy after solution treatment.

ó2012Elsevier B.V.All rights reserved.

1.Introduction

Al–Zn–Mg–Cu alloy has been widely used as structural material in aerospace due to low density,excellent stress corrosion resis-tance and fracture toughness [1].The hot deformation behavior was the basic parameter to characterize plastic deformation prop-erties of alloys,and grain structure evolution during hot deforma-tion affected the strength and fracture toughness [2,3].In recent years,a number of studies on hot deformation behavior and micro-structure evolution of Al–Zn–Mg–Cu alloys have been reported.Jin et al.[4]investigated the hot deformation behavior of 7150Al al-loy,indicating the peak ?ow stress decreased with increasing deformation temperature and decreasing strain rate.Hu et al.[5]reported the main deformation mechanism of 7050Al alloy was grain boundary slip at high Z parameter value while grain bound-aries sliding at low Z parameter value.Zhen et al.[6]demonstrated that the average grain boundary misorientation angle of 7050alu-minum alloy after high-temperature compression increased with an increase deformation temperature.

Moreover,recently researches observed hot deformation behav-iors were signi?cantly affected by heat treatment conditions.Gavgali and Aksakal [7]demonstrated the hot workability of 2014Al alloy increased with increasing homogenization temperature.Totik and Gavgali [8]found that the differences of peak ?ow stress and fracture strain between ingot surface and center were gradually decreased with an increase homogenization temperature.Zhang and Baker [9]reported that 6082Al alloy treated by the T4temper showed distinct softening at the low strain rate while there was no signi?cantly softening under annealing treatment.McQueen et al.[10]revealed that dynamic precipitation increased the initial ?ow stress of Al–Mg–Si alloy and resulted in shear banding and ?ne polygonized substructure.Ebrahimi et al.[11]found that dynamic precipitation and particles coarsen led to ?ow stress softening in the solid solution state while no softening in the annealed state of 2024Al alloy.Cerri et al.[12]reported that pretreatment in?uenced the hot deformation activation energy of 7075Al alloy.The hot deformation activation energy of the precipitated alloy was close to pure aluminum but lower than the solution treated alloy.

7085Al alloy was developed since 2002by Alcoa as the new generation high strength thick plate alloy,which has the higher fracture toughness and lower quench sensitivity than 7050Al alloy [13–15].The hot deformation behavior was a vital factor for the al-loy development and application.However,there was little re-search about hot deformation behavior of the alloy,especially with different heat treatment.The purpose of the study was to

0925-8388/$-see front matter ó2012Elsevier B.V.All rights reserved.https://www.sodocs.net/doc/fe7882763.html,/10.1016/j.jallcom.2012.05.052

Corresponding author.Tel.:+8673188830714;fax:+8673188710855.

E-mail addresses:csuchenkh@https://www.sodocs.net/doc/fe7882763.html, ,khchen@https://www.sodocs.net/doc/fe7882763.html, (K.Chen).

investigate the hot deformation behaviors of7085Al alloy under different heat treatment.The aim was to have the better under-standing the effect of heat treatment on hot deformation behavior of7085Al alloy.

2.Experimental procedure

The materials under study were prepared through the ingot metallurgy route in the laboratory.The raw materials were high purity aluminum(99.9%),magnesium (99.9%)and zinc(99%),and Al–Zr,Al–Cu master alloys were smelted into the alloys. The smelting temperature was kept at700–740°C.Liquid metal was then poured into a mold through semi-continuous casting.The chemical composition of the 7085alloy was determined by inductively coupled plasma(ICP)as shown in Table 1.

Specimens were subjected to homogenization and solution heat treatment.The homogenization treatment consisted:billets were homogenized at450°C for24h and470°C for30h,and slowly cooling to room temperature in air.The solution treatment consisted:homogenization billets were forged and solution treated at 470°C for1h,followed by water quenching.Cylindrical samples with10mm in diameter and15mm in height were machined from homogenization billets and solution treatment https://www.sodocs.net/doc/fe7882763.html,pression tests were carried out on a Gleeble1500 machine at temperatures from300°C to450°C with strain rate from0.0001sà1 to1sà1.The specimens were induction heated to deformation temperature with a heating rate of10°C/s by thermocoupled feedback-controlled AC current and then were held for3min at the deformation temperature in order to obtain a stable and uniform temperature prior to deformation.Subsequently,the specimens were com-pressed to50%reduction,and immediately followed by water quenching to freeze the deformed microstructure.In order to reduce the frictional force between the specimens and the press indenters,a graphite lubricant was used during the iso-thermal compression tests.The microstructures of the deformed specimens were examined by optical microscope and transmission electron microscope(JEOL-2100F).Thin foils for TEM were prepared by mechanical polishing to100l m and ?nal twin-jet electro polishing in a solution of25%HNO3+75%CH3OH atà25°C.

3.Results and discussion

3.1.Initial microstructure

The initial microstructures of the alloy after heat treatment are shown in Fig.1.For the alloy after homogenization treatment,the grain size is about100l m and some coarse particles are distrib-uted along the grain boundaries.For the alloy after solution treat-ment,the grain size is about30l m and there is a little coarse particle along the grain boundaries.

3.2.Flow stress

The?ow stress curves of the alloy after homogenization and solution treatment under various deformation temperatures and strain rates are shown in Figs.2and3,respectively.The results show that,whatever the heat treatment is,the?ow stress in-creases rapidly with strain at the initial stage,and then nearly stea-dy or decreases with strain beyond the peak strain.It is also showed that the?ow stress decreases with a decrease of strain rate and an increase of deformation temperature.It is interesting to note that the?ow stresses of the alloy after solution treatment are higher than the alloy after homogenization treatment.For example,at deformation temperature of300°C and strain rate of 0.1sà1,the peak?ow stress of the alloy after homogenization treatment is135MPa,while the peak?ow stress of the alloy after solution treatment is192MPa,increased42.2%(Fig.3a).The peak ?ow stress of the alloy after solution treatment at deformation temperature of450°C and strain rate of0.1sà1is also higher than the alloy after homogenization treatment(3.8%).Moreover,the ?ow stresses softening(the difference between peak?ow stress and steady state?ow stress)is different under different deforma-tion parameters and heat treatment conditions.For example,the amount of?ow stress softening for the alloy after solution treat-ment is about50MPa under the deformation temperature of 300°C and strain rate of0.0001sà1while the?ow stress softening is only about3MPa under deformation temperature at450°C and strain rate of0.0001sà1.However,there is not signi?cantly?ow stress softening for the alloy after homogenization treatment un-der the same deformation condition.These results show at the low deformation temperature and slow strain rate for the alloy after solution treatment show obviously higher peak?ow stress and?ow stress softening compared to the other deformation parameters or the alloy after homogenization treatment.

3.3.Constitutive equation

The constitutive equations have commonly been applied in high temperature deformation as following[16]:

_e?A1ferTexpeàQ=RTTe1T

where Q is the activation energy for deformation(J/mol),R is the universal gas constant,T is the temperature(K),A1is the constant and f(r)is the stress function.The f(r)can be expressed as following equations[17]:

ferT?r n1e2TferT?expeb rTe3TferT??sin hea rT n2e4Twhere a is the stress multiplier.n1,b,and n2is material constant. The Eq.(2)and Eq.(3)are suitable for high stress and low stress, respectively.The hyperbolic sine law,Eq.(4),is a more general form suitable for stress over a wide range.

Generally,the hyperbolic sine law is found to be the most suit-able form to express the hot deformation behavior of aluminum al-loy[6].Combined with the Eq.(1)and Eq.(4),the following constitutive equation can be obtained:

_e?A2?sin hea rTn expeàQ=RTTe5TThe value of a can also be de?ned as a%b/n1,where b and n1 are taken as the average values of the slopes of the ln_e vs.r plots and ln_e vs.ln(r)plots at a series of temperatures,respectively.

The hot deformation activation energy is an important parame-ter in the plasticity deformation,which is associated with alloy content and heat treatment,etc.[12,18,19].It can be expressed by the following relationship:

Q?R

@ln_e

@ln?sin hea rT

T

@ln?sin hea rT

@e1=TT

_e

?RNSe6T

where N comes from a series of temperatures and is taken as the average values of the slopes of the ln_e and ln[sin h(a r)]plots,while S comes from a series of strain rates and is taken as the average val-ues of the slopes of the ln[sin h(a r)]and1/T plots.

Fig.4presents the liner relationship between ln_e and ln[sin-h(a r)].Fig.5shows the liner relationship between ln[sin h(a r)] and1/T.It can be shown that the peak stress obtained from defor-mation is approximated to a group of parallel and straight lines.

Table2lists the material constants of the alloy under different heat treatment.The hot deformation activation energy of homoge-nized alloy is187.4kJ/mol,which is lower than the homogenized 7150alloy(229.75kJ/mol)[5],higher than the aged7150alloy (158.8kJ/mol)and the over aged7075alloy(143–156kJ/mol) [20].The value of the hot deformation activation energy for the

Table1

Chemical composition of the AA7085.

Element Zn Mg Cu Zr Al

Wt.%7.5 1.6 1.50.12Bal.

S.Chen et al./Journal of Alloys and Compounds537(2012)338–345339

solution treated7085alloy is close to that of solution treated7075 alloy(300–400kJ/mol),is much higher than and the solution trea-ted7012alloy(200–230kJ/mol)[12].

The hot deformation behavior of material can be represented by the Zener–Hollomon parameter(Z).The expression of Z parameter is obtained as:

Z?_e expeàQ=RTTe7TFig.6shows the relationship between ln(Z)and ln[sin h(a r)]of the alloy under different heat treatment.The Z parameter values increase with decreasing deformation temperature and increasing strain rate.These demonstrate that the peak stresses of the alloy under different heat treatment can be represented by Zener–Hollo-man parameters in hyperbolic sine-type equations.

3.4.Microstructure evolution

The optical microstructures of the two treated alloys under dif-ferent deformation conditions are shown in Figs.7and8.For the alloy after homogenization treatment,the deformed grains are elongated and without recrystallization at deformation tempera-ture of350°C(Fig.7a and b).While deformation temperature at 450°C,recrystallized grains can be observed,indicating that the dynamic recrystallization occurs at high deformation temperature. However,for the alloy after solution treatment,the deformed microstructures are different with the alloy after homogenization treatment.The deformed grain size is smaller than that of the alloy after homogenization treatment under similar deformation condi-tions.It is also observed that the alloy after solution treatment shows the higher volume fraction of recrystallized grains than those of the alloy after homogenization treatment.

TEM microstructures of the alloy after homogenization treat-ment under different deformation conditions are shown in Fig.9. The high density of dislocations tangles with large precipitates at deformation temperature of350°C and strain rate of1sà1 (Fig.9a).With the strain rate decreases to0.001sà1,the dislocation density slightly decreases and the low angle grain boundaries are transformed into high angle grain boundaries(Fig.9b).When the deformation temperature increases to450°C,the dislocations annihilate and the subgrain boundaries become straight at strain

The initial microstructure of the alloy after different heat treatment(a)homogenization treatment;(b)solution treatment. of the7085Al alloy after homogenization treatment under different deformation temperatures:(a)300°C,(b)350°C,

curves of the7085Al alloy after solution treatment under different deformation temperatures:(a)300°C,(b)350°C,(c) Relationship between strain rate and?ow stress ln_eàln[sin h(a r)]):(a)homogenization treatment;(b)solution treatment.

5.Relationship between?ow stress and deformation temperature:(a)homogenization treatment;(b)solution treatment.

rate of1sà1(Fig.9c)while the subgrains grow at strain rate of à1dispersion of?ne precipitates was responsible for high stress at the low deformation temperatures(e.g.,350°C),and the precipi-tates coarsen and dynamic recovery led to the?ow stress softening with strain at the low deformation temperatures(Fig.3a and b). However,the?ow stress softening with an increase of strain at the high deformation temperature(e.g.,450°C)was mainly related to dynamic recovery and recrystallization.

The deformed microstructures of the alloy after homogeniza-

Table2

Values of constitutive constants of the alloy with different heat treatment.

Condition A a n Q(kJ/mol)

Homogenization 6.39096?10110.01603 6.259187.4

Solution7.45327?10210.01742 5.66735318.9

Relationship between Zener–Hollomon parameter(Z)and?ow stress:(a)homogenization treatment;(b)solution treatment.

the7085Al alloy after homogenization treatment under different deformation conditions:(a)T=350°C,_e?1sà1;(b)T

=450°C,_e?0:001sà1.

342S.Chen et al./Journal of Alloys and Compounds537(2012)338–345

supersaturation solid solution of the alloy after solution treatment. The amount,size and distribution of the precipitates were depen-dent on the deformation temperature and strain rate.Fine precip-itates have been precipitated at the low deformation temperature (e.g.,350°C)while little?ne precipitates have been formed at the high deformation temperature(e.g.,450°C)(Fig.3c and d).

The alloy after solution treatment show higher?ow stress and hot deformation activation energy than those of the alloy after homogenization treatment.The reasons can be explained from the following aspects:(i)dynamic precipitation.For the alloy after solution treatment,a large number of?ne precipitates have been dynamic precipitated at low deformation temperature.These

7085Al alloy after solution treatment under different deformation conditions:(a)T=350°C,_e?1sà1;(b)T=350°C,_e ?0:001sà1.

alloy after homogenization treatment under different deformation conditions:(a)T=350°C,_e?1sà1;(b)T=350°C,e ?0:001sà1.

uniform ?ne precipitates pinned the dislocations mobility effec-tively and reduced dynamic recovery.For the alloy after homogeni-zation treatment,however,the precipitation strength was not

obviously due to little ?ne precipitates.(ii)Residual solute remain-ing in the solid solution.There were more residual solute elements in the matrix of solution treatment alloy than the homogenized treatment alloy under the same deformation condition.The solute elements exerted a substantial back force on the dislocations and signi?cantly in?uenced the ?ow stress.However,the effects were decreased as precipitates proceeds with an increase strain and the decease deformation temperature.It was suggested that the stress increment of solutes in the solid solution was limited.(iii)Grain size.The stress increment from the grain re?nement should be ta-ken into consideration.The grain size of the alloy after solution treatment were smaller than those of homogenized alloys under the same deformation condition due to small initial grains size.But the stress increment could be neglected compare with precipi-tation strength.From the above explanations,it can be concluded that the dynamic precipitation of the alloy after solution treatment was mainly responsible for the higher ?ow stresses compared with the alloy after homogenization treatment.These explanations were also in correspondence with higher hot deformation activation en-ergy of the alloy after solution treatment than that of the alloy after homogenization treatment.These phenomenons have been ob-served in 6082Al alloy [9],6201Al alloy [10]and 2024Al alloy [11].From the above experiments and explanations,it is obvious that the ?ow stress of the alloy after solution treatment is signi?cantly higher than that of the alloy after homogenization treatment under the similar compression condition,especially at low deformation temperature.It can be concluded that the dynamic precipitation is taken disadvantage to hot deformation.Therefore,the dynamic precipitation during the deformation process is taken disadvantage to hot deformation.From industrial production point of view,the preheat and annealing temperature should below the solute temperature in order to avoid high supersaturated solid solution of aluminum matrix and dynamic precipitation under the low deformation temperature.The optimal processing parameter of 7085aluminum alloy under different heat treatment was

corresponds to a deformation temperature of 723K and strain rate of 0.0001s à1from the ?ow stress.4.Conclusions

The hot deformation behavior of 7085aluminum alloy with dif-ferent heat treatment has been studied by isothermal compression in the temperature range from 300°C to 450°C with the strain rate from 0.0001s à1to 1s à1.On the basis of the stress strain behavior,microstructure examination,the following conclusions are draw:

1.The ?ow stress for both homogenized and solution treated

7085aluminum alloy increases with decreasing strain rate and increasing deformation temperature.

2.The peak ?ow stress can be represented by the Zener–

Hollomon parameter (Z )in the hyperbolic-sine equation and the hot deformation activation energy for the alloys after homogenization treatment and solution treatment is 187.4kJ/mol and 318.9kJ/mol,respectively.

3.The peak ?ow stresses for the alloy after the solution treat-ment are higher than the alloy after homogenized treat-ment under the similar compression condition,especially low deformation temperature.The dynamic precipitation of the alloy after solution treatment is mainly responsible for the higher ?ow stresses compared with those of the alloy after homogenization treatment.

4.The dynamic precipitation and precipitation coarsening as

well as dynamic recovery are responsible for the ?ow softening at the low deformation temperature,and the dynamic recovery and recrystallization are the main reasons for the ?ow softening at the high deformation temperature of the alloy after solution treatment.

Acknowledgements

The authors wish to acknowledge the ?nancial support of National Basic Research Program of China (Nos.

2010CB731701

Al alloy after solution treatment under different deformation conditions:(a)T =300°C,_e

?1s à1;(b)T =350°C,_e C,_e ?0:001s à1.

and2012CB619502),Hunan Provincial Innovation Foundation for Postgraduate,and Creative research group of National Natural Science Foundation of China(No.51021063).

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